This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to an apparatus for providing improved registration of the beam in the process direction movement of the photoconductive member.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer uses a raster output scanner (ROS) to expose the charged portions of the photoconductive member to record the electrostatic latent image thereon. Generally, a raster output scanner has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
Laser printers may operate in either a single pass or multiple pass system. In a single pass, color xerographic system, three ROS stations are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS station may be added if black images are to be created as well. In a multiple pass system, each image area on the photoreceptor surface must make at least three revolutions (passes) relative to the transverse scan line formed by the modulated laser beam generated by a ROS system. With either system, each image must be superimposed (registered) to within a 0.1 mm circle or within a tolerance of .+-.0.05 mm. Each color image must be registered in both the photoreceptor process direction (skew registration) and in the direction perpendicular to the process direction (referred to as the fast scan or transverse registration).
Various techniques are known in the prior art for registering multiple image exposures in both the transverse and process direction. Copending U.S. application, Ser. Nos. 07/635,835 and 07/807,927, both assigned to the same assignee as the present invention, disclose two of such techniques. The contents of these applications are hereby incorporated by reference. The Ser. No. 07/807,927 application discloses a preferred embodiment of a ROS scanning system to which the present invention has utility. FIG. 1 shows a prior art, single pass, ROS color printing system 8 having four ROS systems, 10, 12, 14, and 16. The system 8 includes a photoreceptor belt 18, driven in the process direction, indicated by the arrow 19. The length of the belt 18 is designed to accept an integral number of spaced image areas I.sub.1 -I.sub.n represented by dashed line rectangles in FIG. 1. Upstream of each image area is a charging station (not shown) which places a predetermined electrical charge on the surface of belt 18. As each of the image areas I.sub.1 -I.sub.n reaches a leading edge transverse line of scan, represented by lines 20a -20d, a lead edge signal is generated by a belt hole/sensor and the image area is progressively exposed on closely spaced transverse raster lines 22, shown with exaggerated longitudinal spacing on the image area I.sub.4 in FIG. 1. Each image area I.sub.1 -I.sub.n is exposed successively by ROS systems 10, 12, 14, 16. Downstream from each exposure station, a development station (not shown) develops the latent image formed in the preceding image area. A fully developed color image is then transferred to an output sheet. Details of charge and development xerographic stations in a multiple exposure single pass system are disclosed, for example, in U.S. Pat. Nos. 4,660,059 and 4,611,901, whose contents are hereby incorporated by reference. The charge, development, and transfer stations are conventional in the art. Each ROS system contains its own conventional scanning components, of which only two, the laser light source and the rotating polygon, are shown. The particular system 10 has a gas, or preferably, laser diode 10a, whose output is modulated by signals from control circuit 30 and optically processed to impinge on the facets of rotating polygon 10b. Each facet reflects the modulated incident laser beam as a scan line, which is focused at the photoreceptor surface. Control circuit 30 contains the circuit and logic modules which respond to input video data signals and other control and timing signals to operate the photoreceptor drive synchronously with the image exposure and to control the rotation of the polygon 10b by a motor (not shown). The other ROS systems 12, 14, 16, have their own associated laser diodes 12a, 14a, 16a, and polygons 12b, 14b, 16b, respectively. In the system of FIG. 1, transverse alignment of each successive image exposure is obtained by providing horseshoe shaped sensor/light emitter units 36a, 36b, 36c, 36d, which cooperate with apertures T1, T2, T3, T4, respectively, formed in the belt surface to provide the lead edge signal. The top part of unit 36 contains a light source and the bottom leg contains a sensor. Further details regarding transverse alignment registration are described in the aforementioned application Ser. No. 07/635,835.
The above-described techniques can result in a registration error of up to one scan line spacing. This is due to the fact a lead edge signal generated by a hole/sensor is dependent upon the instantaneous position of the rotating polygon or, more particularly, the position of the scan beam on the instantaneous scanning facet. The drive control for operation of the laser is delayed until the beam is swept across the instant facet on the scan line and the lead edge scan line is written with arrival of the next facet. Thus, the worst case lead edge misregistration is approximately one scan line spacing. As an example, for a scanner having resolution of 400 spots per inch (spi), the error could be 0.0025 inch. This error could be minimized by increasing the resolution of the system, thereby decreasing the scan line spacing and hence the maximum misregistration. However, this is an expensive solution and, given the state of the art, even 600 spi systems are difficult to achieve.
The present invention is directed towards reducing the lead edge registration error by a factor of 2 so that, for the example given above of a 400 spi system, the maximum lead edge registration error would 0.00125 inch. This is realized by providing detection and control circuitry which identifies the beam position on the polygon facet at the time the leading edge sensor generates an output signal. Discrimination circuitry determines whether the beam is before or after the mid-scan position on the instantaneous scanning facet and enables the leading edge start-of-scan to occur beginning with the next facet (if the beam is past the midpoint position) or to discard the scan line formed when the beam is prior to the midpoint position and to wait for the next facet to begin the lead edge scan. With either event, the error is less than 1/2 scan line spacing with a maximum error occurring when the beam is exactly at the midpoint position.
More particularly, the present invention relates to an imaging system for forming multiple image exposure frames on a photoconductive member during a single pass including:
a photoreceptor belt adapted to accommodate the formation of an integral number of image exposure frames,
a plurality of Raster Output Scanners (ROS) units, each ROS unit forming one of said image exposure frames, each ROS unit projecting a plurality of scanning beams in a fast scan (transverse) direction across the belt width, by scanning modulated beams from the facet surfaces of a rotating polygon, each facet having a width W,
start of scan (SOS) and end of scan (EOS) sensor means for controlling the start and end of the scanning beams,
belt position detector means for detecting the location of the leading edge of an exposure frame and for generating an output signal to the ROS unit to write the first line of the image frame, and
control circuitry for detecting the instantaneous scanning beam position on the instantaneous scanning facet at the time of generation of said belt position detector means output signal and for controlling said ROS unit so as to initiate the first image scan line at the next SOS sensing if the beam position has been found at a distance along the facet greater than W/2 and to initiate the second image scan line at the next SOS sensing if the beam position is at a distance along the facet which is less than W/2.